1. Field of the Invention
The invention relates to a stimulation electrode having a porous surface coating whose active surface area is essentially larger than the surface area defined by the geometrical basic shape of the electrode.
2. Description of the Related Art
From an electrical standpoint, the phase limit between a solid body, that is, between the stimulation electrode of a pacemaker and an electrolyte, can be described in a simplified manner as a parallel switching of the phase limit capacity, i.e. the Helmholtz capacity C.sub.H, and the Faraday resistance R.sub.F, with which the line resistance R.sub.L is connected in series. The impedance of the electrode system Z.sub.DL is a function of the frequency .omega. of the applied voltage in accordance with the following equation: ##EQU1## A specified charge Q is required to stimulate the heart muscle, which charge can be calculated from the integral of the stimulation current I(t) over the pulse width T. Consequently, the impedance of the electrode system Z.sub.DL can likewise be minimized when the quantity of energy required for stimulating the heart is minimized: ##EQU2## Because the line resistance R.sub.L is constant, the Faraday resistance R.sub.F is defined with the following equation ##EQU3## where R.sub.o is a constant transfer resistance and A is the active surface area, and the Helmholtz capacity C.sub.H is defined as follows: ##EQU4## where .epsilon. is the dielectric constant of the added water dipoles, .epsilon..sub.o is the dielectric constant of the vacuum and d is the thickness of the Helmholtz layer, the enlargement of the active surface area of the electrode according to (3) leads to an increase in the Helmholtz capacity C.sub.H and, according to (2), to a reduction in the Faraday resistance R.sub.F. According to (1), both then have as a consequence a reduction in the impedance Z.sub.DL and in the required quantity of energy E. The active surface area A can be changed particularly by the enlargement in the electrode and/or by structuring of the electrode surface area.
Stimulation electrodes whose electrochemically active surfaces are enlarged by means of a porous layer composed of a carbide, nitride or carbon nitride, at least one of the metals including titanium, vanadium, zirconium, niobium, molybdenum, hafnium, tantalum or tungsten are already known from EP-A-0,117,972, EP-A-0,116,280 and EP-A-0,115,778.
A drawback of these known porous electrode coatings is that the overall capacity of the implanted electrodes decreases slowly over time, resulting in a corresponding increase in the required quantity of energy. Hence, the stimulation voltage must be selected relatively high in order to exceed the threshold response of the patient with the pulse energy over the long term. However, to emit the increased energy, an increase in the voltage of the pulses, which again results in an enlargement of the energy sources--and thus an enlargement in the housing--in implanted systems. The increase in pulse energy is accompanied by an increase in the polarization voltage, so that, after completed stimulation, the conventionally employed counterpulses must be correspondingly increased to prevent the effects of the polarization voltage on the input amplifier of the pacemaker.
It is an object of the invention to improve a stimulation electrode of the above-mentioned type in such a way that, on the one hand, the energy required for stimulation can also remain low in the long term and, on the other hand, that a reliable recognition of effectiveness is assured with simple measures.